1. Field of the Invention
The present invention relates to the field of recreational vehicle fabrication and, in particular, to methods of prefabricating a motorhome frame and then joining the frame to a prefabricated vehicle chassis to provide a motorhome with increased structural strength and interior ceiling height with improved production efficiency and reduced cost and time of production.
2. Description of the Related Art
Motorhomes have become an increasingly popular and common means of recreation. Motorhomes are self-propelled vehicles that include a living space inside. Motorhomes typically provide sleeping areas, cooking facilities, and self-contained water supplies and toilet facilities. More elaborate motorhomes can include refrigerator/freezer units, showers and/or bathtubs, air conditioning, heaters, built in generators and/or power inverters, televisions, VCRs, and clothes washers and dryers. Motorhomes provide many of the amenities of a residential home while on the road away from home and are popular for this reason. Motorhome users will typically use the motorhome to travel to a recreational area and live in the motorhome for some period of time. It is not unusual for people, particularly retired persons, to use a motorhome as their primary residence. Motorhome users often have families with children and, as their trips are often of a recreational nature, will often invite friends or family along on the trip.
It can be understood that since a motorhome will often be used by a large number of people and often for an extended period of time, the motorhome manufacturers and customers will seek as many amenities and as much interior living space as possible. A major goal of motorhome manufacturers and their customers is to maximize the amount of usable living space inside their motorhomes. However, the overall size of an motorhome is limited both by vehicle code regulations and by practical limitations on what is reasonable to drive and maneuver. Vehicle codes restrict the maximum height, width, and length of vehicles that may be driven on public roads. Also, as a vehicle increases in size, it becomes increasingly difficult to drive and can become physically too large to pass through locations that the driver may wish to go. In addition, as the motorhome gets physically larger, more fuel is required to move it, which increases the cost of operation. Accordingly, many motorhomes are provided with slide-out room structures which are extendable so as to increase the motorhome's footprint and interior living space once the vehicle comes to rest.
An additional design constraint on the construction and design of motorhomes is their overall weight. Since an motorhome is intended to be mobile, an integral power plant is provided and the engine and drive-train have an upper design limit on the weight it is capable of moving. In addition, the chassis, suspension, wheels, and brakes of a motorhome also have upper design limits as to how much weight they can safely accommodate. These weight limits are established after careful engineering analysis and the weight ratings are endorsed and enforced by responsible governmental agencies. Exceeding the established weight limits of a power-train or chassis component can lead to excessive wear and failure, unacceptable performance, and exposure to liability in case of an accident. It is also highly desirable that as much payload as possible is available to accommodate passengers and cargo, i.e. available weight load between the wet weight of the motorhome and the total maximum gross weight of the motorhome.
A particular issue with the weight of a motorhome is its distribution along a vertical axis. The distance of the vehicle's center of mass from the road surface has a dramatic effect on the handling characteristics of the vehicle. The closer the center of mass is to the road surface, the shorter the moment arm between the center of mass and the roll axis of the vehicle. The shorter the moment arm between the center of mass and the roll axis of the vehicle, the less tendency the vehicle will have to lean in turns. Leaning in turns is uncomfortable for the occupants and typically places uneven loads on the tires and suspensions, compromising turning ability. Motorhomes, typically being quite tall, often exhibit significant leaning in turns. However, within the height available in a motorhome, the weight should be concentrated as low as possible. For this reason, heavy items, such as generators, storage and holding tanks for water and fuel, and the engine are optimally placed low in the chassis.
Since motorhomes are mobile structures, they are typically exposed to the stresses of driving over roads that are in places quite rough. In addition, an motorhome will often have to travel over some distance of dirt surface to reach a camping space. Since an motorhome is typically used outdoors, it is exposed to the stresses of inclement weather and high winds. It can be appreciated that structural integrity is highly desired in an motorhome. However, the weight and size limitations previously mentioned place a limit on the strength of an motorhome. Accordingly, motorhomes are constructed to be as strong, but as light as possible.
The chassis of a motorhome is typically constructed on a steel ladder frame chassis. The chassis is a partially complete vehicle and is generally procured from a manufacturer such as Freightliner or Ford Motor Company. The chassis typically consists of two parallel frame rails extending the length of the chassis and interconnected with several perpendicular cross-braces to form a ladder frame. An engine, transmission, and fuel tank(s) are generally placed between the frame rails near one end. Suspension, steering, brake, and road wheel assemblies are attached outboard of the frame rails.
The coach bodywork, which provides and encloses the living space of the motorhome, is typically made from a laminate that can include light gauge sheet metal, plywood, vinyl, and insulation. The laminate is built to be strong, lightweight, weather resistant, and durable. The coach bodywork may also include a supporting framework. The floor of the coach typically rests indirectly on the chassis frame and the vertical walls extend upwards from the floor. The roof of the coach rests on and depends on the vertical walls of the body for structural support.
A completed motorhome may be up to 45′ long and 13′6″ high in most states. The chassis is generally on the order of 1′ high and is elevated some distance above the ground by the suspension and wheels to provide ground clearance for suspension movement and clearing obstacles in the road. The interior flooring in current art motorhomes is typically elevated a significant amount above the upper face of the chassis in order to facilitate installing ancillary equipment. In addition, many prior art motorhomes route cooling or heating air ducts adjacent the roof structure or mount air-conditioning units on the roof. Under the overall height limit previously mentioned, these structures in or on the roof intrude into the available interior height envelope and limit the usable interior vertical space. Current motorhomes typically offer interior ceiling heights of 6′9″ or less. The slide-outs in current art motorhomes do not typically provide sufficient room inside for adults to stand upright. As the slide-out area is a living space in the extended position, it can be appreciated that to be forced to stoop or crouch inside the slide-out is an inconvenience for the users of such motorhomes.
An additional difficulty arises with motorhomes of the construction described above when the vehicle drives over rough terrain. Motorhomes are essentially rectangular and are thus susceptible to twisting deflection as opposed to a triangulated structure such as a trailer or a fifth wheel. Motorhomes of the construction described above are not particularly strong under torsional forces such as arise when one corner of the vehicle drops or rises compared to the others, for example when a wheel drops into a pothole or rut or the vehicle drives over a curb or speed bump at an angle. Such maneuvers “cross-up” the rectangular frame and impose twisting forces. These forces, exerted over the relatively large dimensions of a typical motorhome, can cause significant displacement in the coach. This can cause cracks to appear in the coach, jam door and window openings, dislodge interior fitments, and generally cause wear and tear on the vehicle.
Furthermore, as stated, a motorhome is typically constructed on an unfinished vehicle chassis procured from a separate manufacturer, such as Freightliner or Ford Motor Company. As such, the motorhome manufacturer is dependent on delivery of the preassembled chassis before motorhome fabrication can begin. This results in a series production that has increased assembly time as compared to a parallel process expensive to purchase and inventory.
From the foregoing, it can be appreciated that there is a continuing need for a stronger motorhome coach construction that also provides increased interior living space. The structure should minimize weight to the motorhome and should also maintain as low a center of gravity as possible to benefit vehicle handling characteristics. There is also a need for a method of fabricating the motorhome with increased efficiency and reduced cost and construction time.